An LED (light-emitting diode) consists of a semiconductor junction, which emits light due to a current flowing through the junction. An LED light bulb is a device that contains one or more LEDs and potentially a drive circuit for the LEDs, which are both located inside a conventionally shaped container (or shell). Conventional LED light bulbs screw into a standard socket and receive AC power from the two power connections of the socket.
However, 3-way sockets differ from standard sockets in that they have three power connections. The control of the socket is configured so that, in sequence, a first power configuration (i.e., a first pair) consisting of the base and one of the power connections, which is energized with AC power, a second power configuration (i.e., a second pair) consisting of the base and a second power connection which is energized, and a third power configuration, which includes the base and both connections (i.e., the first and the second power connections), which are energized. A conventional LED light bulb will be energized only in one of the configurations, and will not respond with different light levels to the different settings.
It can be appreciated that one solution to this problem is to have separate driving circuits energized by each of the configurations (or pairs). It can be appreciated that the circuits can be designed to deliver different drive power to the LEDs, and to deliver the sum of these two drive powers when both are energized. However, in practice, this solution may be expensive, and very difficult to fit inside the bulb.